Suction conduit flow control for lubricant management

ABSTRACT

A system includes first and second compressors arranged in parallel, a condenser, expansion device, evaporator, and flow control device fluidly connected. The first compressor includes a first lubricant sump and the second compressor including a second lubricant sump. A lubricant transfer conduit fluidly connects the first lubricant sump and the second lubricant sump. The flow control device is disposed between the evaporator and the first and second compressors, and includes a fluid inlet and two fluid outlets. A first of the two fluid outlets is fluidly connected to the first compressor, a second of the two fluid outlets is fluidly connected to the second compressor. The second fluid outlet includes a nozzle disposed within a flow passage of the flow control device such that a space is maintained between an outer surface of the nozzle and an inner surface of the flow passage.

FIELD

This disclosure relates generally to heating, ventilation, airconditioning, and refrigeration (HVACR) systems. More specifically, theembodiments relate to lubricant management between a plurality ofcompressors connected in parallel.

BACKGROUND

A heat transfer circuit for a heating, ventilation, air conditioning,and refrigeration (HVACR) system generally includes a compressor, acondenser, an expansion device, and an evaporator fluidly connected. Insome heat transfer circuits, a plurality of compressors can be connectedin parallel. To ensure sufficient lubricant is supplied to bothcompressors, a lubricant level switch can be included in a lubricantsump of one or more of the compressors. When the lubricant level fallsbelow a threshold of the lubricant level switch, operation of one ormore of the plurality of compressors (e.g., speed modification if thecompressor is a variable speed compressor; and/or starting/stopping thecompressor if it is a fixed speed compressor) may be modified in orderto bring the lubricant level back above the threshold. This can, forexample, prevent the plurality of compressors from being operated withan insufficient supply of lubricant, but can impact temperature controlof the heat transfer circuit due to the modification of the operation ofthe one or more compressors.

SUMMARY

This disclosure relates generally to heating, ventilation, airconditioning, and refrigeration (HVACR) systems. More specifically, theembodiments relate to lubricant management between a plurality ofcompressors connected in parallel.

The plurality of compressors includes a motor-compressor unit includinga lubricant sump. In some embodiments, the lubricant sump is disposed ata relatively vertically lower portion of the compressor such thatlubricant can collect in the lubricant sump via gravitational force. Insome embodiments, the lubricant is entrained in a heat transfer fluid ofa heat transfer circuit of the HVACR system. The lubricant isaccordingly provided to the plurality of compressors via a suctionconduit which provides gaseous heat transfer fluid from an evaporator ofthe heat transfer circuit to the plurality of compressors.

In some embodiments, the plurality of compressors can include twocompressors. A first of the two compressors can be referred to as the“upstream compressor.” The other of the two compressors can be referredto as the “downstream compressor.” In some embodiments, the upstreamcompressor can be a variable speed compressor and the downstreamcompressor can be a fixed speed compressor.

In some embodiments, the plurality of compressors can include more thantwo compressors. In some embodiments, the plurality of compressors caninclude three compressors. In some embodiments, the plurality ofcompressors can include four compressors. In some embodiments, theplurality of compressors includes at least one variable speedcompressor.

A flow control device can be disposed between the evaporator and theplurality of compressors. The flow control device can be designed tocontrol a flow of heat transfer fluid and lubricant to each of the twocompressors.

In some embodiments, the flow control device can separate the gaseousheat transfer fluid of the suction conduit into a lubricant rich portionand a lubricant free portion. In some embodiments, the lubricant richportion of the gaseous heat transfer fluid can be provided to a suctioninput of the upstream compressor. In some embodiments, the lubricantfree portion of the gaseous heat transfer fluid can be provided to asuction input of the downstream compressor.

In some embodiments, the flow control device can be designed to controla differential pressure between the upstream compressor and thedownstream compressor. Controlling the differential pressure between theupstream compressor and the downstream compressor can, for example,ensure that excess lubricant flows from the lubricant sump of theupstream compressor to the lubricant sump of the downstream compressor.

A system is disclosed. The system includes first and second compressors,a condenser, an expansion device, an evaporator, and a flow controldevice fluidly connected. The first and second compressors are arrangedin parallel. The first compressor includes a first lubricant sump andthe second compressor including a second lubricant sump. A lubricanttransfer conduit fluidly connects the first lubricant sump and thesecond lubricant sump. The flow control device is disposed between theevaporator and the first and second compressors, and includes a fluidinlet and two fluid outlets. A first of the two fluid outlets is fluidlyconnected to the first compressor and a second of the two fluid outletsis fluidly connected to the second compressor. The second fluid outlethaving a nozzle disposed within a flow passage of the flow controldevice such that a space is maintained between an outer surface of thenozzle and an inner surface of the flow passage.

A method is disclosed. The method includes separating a flow of a heattransfer fluid and lubricant mixture into a lubricant rich portion and alubricant free portion. The method further includes directing thelubricant rich portion to a suction inlet of a first compressor, thefirst compressor being a variable speed compressor; and directing thelubricant free portion to a suction inlet of a second compressor, thesecond compressor being a fixed speed compressor. The first and secondcompressors are arranged in parallel in a heat transfer circuit and thefirst compressor is upstream of the second compressor.

BRIEF DESCRIPTION OF THE DRAWINGS

References are made to the accompanying drawings that form a part ofthis disclosure and which illustrate embodiments in which the systemsand methods described in this specification can be practiced.

FIG. 1 is a schematic diagram of a heat transfer circuit, according tosome embodiments.

FIG. 2 is a sectional view of a flow control device for use in the heattransfer circuit of FIG. 1, according to some embodiments.

FIGS. 3A-3E illustrate various views of an adapter for a lubricanttransfer conduit, according to some embodiments.

Like reference numbers represent like parts throughout.

DETAILED DESCRIPTION

This disclosure relates generally to heating, ventilation, airconditioning, and refrigeration (HVACR) systems. More specifically, theembodiments relate to lubricant management between a plurality ofcompressors connected in parallel.

A “lubricant rich portion,” as used in this specification, includes aportion of a heat transfer fluid and refrigerant mixture that has arelatively higher concentration of lubricant compared to another portionof the heat transfer fluid flow.

A “lubricant free portion,” as used in this specification, includes aportion of a heat transfer fluid and refrigerant mixture that has arelatively lower concentration of lubricant compared to another portionof the heat transfer fluid flow. It will be appreciated that thelubricant free portion may still include some lubricant.

FIG. 1 is a schematic diagram of a heat transfer circuit 10, accordingto some embodiments. The heat transfer circuit 10 generally includes aplurality of compressors 12A, 12B, a condenser 14, an expansion device16, and an evaporator 18. The heat transfer circuit 10 is exemplary andcan be modified to include additional components. For example, in someembodiments the heat transfer circuit 10 can include other componentssuch as, but not limited to, an economizer heat exchanger, one or moreflow control devices, a receiver tank, a dryer, a suction-liquid heatexchanger, or the like.

The heat transfer circuit 10 can generally be applied in a variety ofsystems used to control an environmental condition (e.g., temperature,humidity, air quality, or the like) in a space (generally referred to asa conditioned space). Examples of systems include, but are not limitedto, heating, ventilation, air conditioning, and refrigeration (HVACR)systems, transport refrigeration systems, or the like.

The components of the heat transfer circuit 10 are fluidly connected.The heat transfer circuit 10 can be specifically configured to be acooling system (e.g., an air conditioning system) capable of operatingin a cooling mode. Alternatively, the heat transfer circuit 10 can bespecifically configured to be a heat pump system which can operate inboth a cooling mode and a heating/defrost mode.

Heat transfer circuit 10 can operate according to generally knownprinciples. The heat transfer circuit 10 can be configured to heat orcool a heat transfer fluid or medium (e.g., a liquid such as, but notlimited to, water or the like), in which case the heat transfer circuit10 may be generally representative of a liquid chiller system. The heattransfer circuit 10 can alternatively be configured to heat or cool aheat transfer medium or fluid (e.g., a gas such as, but not limited to,air or the like), in which case the heat transfer circuit 10 may begenerally representative of an air conditioner or heat pump.

In operation, the compressors 12A, 12B compress a heat transfer fluid(e.g., refrigerant or the like) from a relatively lower pressure gas toa relatively higher-pressure gas. The relatively higher-pressure andhigher temperature gas is discharged from the compressors 12A, 12B andflows through the condenser 14. In accordance with generally knownprinciples, the heat transfer fluid flows through the condenser 14 andrejects heat to a heat transfer fluid or medium (e.g., water, air,etc.), thereby cooling the heat transfer fluid. The cooled heat transferfluid, which is now in a liquid form, flows to the expansion device 16.The expansion device 16 reduces the pressure of the heat transfer fluid.As a result, a portion of the heat transfer fluid is converted to agaseous form. The heat transfer fluid, which is now in a mixed liquidand gaseous form flows to the evaporator 18. The heat transfer fluidflows through the evaporator 18 and absorbs heat from a heat transfermedium (e.g., water, air, etc.), heating the heat transfer fluid, andconverting it to a gaseous form. The gaseous heat transfer fluid thenreturns to the compressors 12A, 12B. The above-described processcontinues while the heat transfer circuit 10 is operating, for example,in a cooling mode (e.g., while the compressors 12A, 12B are enabled).

The compressors 12A, 12B can be, for example, but are not limited to,scroll compressors. In some embodiments, the compressors 12A, 12B can beother types of compressors. Examples of other types of compressorsinclude, but are not limited to, reciprocating compressors, positivedisplacement compressors, or other types of compressors suitable for usein the heat transfer circuit 10 and having a lubricant sump. Thecompressor 12A is generally representative of a variable speedcompressor and the compressor 12B is generally representative of a fixedspeed compressor. In some embodiments, the compressors 12A, 12B canalternatively be step control compressors (e.g., compressors having twoor more steps within a compressor). In some embodiments, the compressors12A, 12B can be compressors having different capacities. For example,compressor 12A can have a relatively greater capacity than compressor12B, according to some embodiments. It will be appreciated thatalternatively the compressor 12B can have a relatively greater capacitythan compressor 12A. In some embodiments, the compressor 12A canalternatively be referred to as the “upstream compressor” and thecompressor 12B can alternatively be referred to as the “downstreamcompressor.”

The compressors 12A, 12B are connected in parallel in the heat transfercircuit 10. Accordingly, the gaseous heat transfer fluid exiting theevaporator 18 is provided via a suction conduit 22 to each of thecompressors 12A, 12B. A flow control device 20 receives the gaseous heattransfer fluid at a fluid inlet 24 and provides the gaseous heattransfer fluid to the compressor 12A via a first fluid outlet 26 and tothe compressor 12B via a second fluid outlet 28. The flow control device20, according to some embodiments, is discussed in additional detail inaccordance with FIG. 2 below. Following compression, the relativelyhigher-pressure and higher-temperature gas is discharged from compressor12A via discharge conduit 32A and from compressor 12B via dischargeconduit 32B. In some embodiments, the discharge conduits 32A, 32B of thecompressors 12A, 12B are joined at discharge conduit 34 to provide thecombined relatively higher-pressure and higher temperature gas to thecondenser 14.

The heat transfer fluid in the heat transfer circuit 10 generallyincludes a lubricant entrained with the heat transfer fluid. Thelubricant is provided to the compressors 12A, 12B to lubricate bearingsand seal leak paths of the compressors 12A, 12B. When the relativelyhigher-pressure and higher-temperature heat transfer fluid is dischargedfrom the compressors 12A, 12B, the heat transfer fluid generally carriesalong with it a portion of the lubricant which is initially delivered tothe compressors 12A, 12B with the heat transfer fluid that enters thecompressors 12A, 12B via a suction conduit 22. A portion of thelubricant is maintained in the lubricant sumps 13A, 13B of thecompressors 12A, 12B.

The lubricant sumps 13A, 13B of the compressors 12A, 12B are fluidlyconnected via a lubricant transfer conduit 36. The lubricant transferconduit 36 is disposed at a lubricant level of the lubricant sumps 13A,13B which permits lubricant to flow between the compressor 12A and thecompressor 12B. Fluid flow of the lubricant is controlled by a pressuredifferential between the lubricant sump 13A of the upstream compressor12A and the downstream compressor 12B. As a result, if operation of thecompressor 12A or 12B is modified, the fluid flow of the lubricantbetween the compressors 12A, 12B can be affected. The flow controldevice 20 is designed such that a desired pressure differential ismaintained between the upstream compressor 12A and the downstreamcompressor 12B. In some embodiments, the desired pressure differentialcan be selected such that flow of lubricant in the lubricant sump 13A isinduced to lubricant sump 13B at a variety of compressor 12A, 12Boperating conditions. In some embodiments, the desired pressuredifferential can alternatively be referred to as a target pressuredifferential. In some embodiments, the desired pressure differential canbe a minimum pressure differential at which flow of lubricant from thelubricant sump 13A will be induced to the lubricant sump 13B. In someembodiments, the desired pressure differential can be a minimum pressuredifferential where flow to upstream compressor 12A can be defined at amaximum compressor speed and flow to downstream compressor 12B can bedefined at a minimum suction flow corresponding to a low suctiontemperature. The low flow to downstream compressor 12B can be when theflow control device 20 is at its minimum effectiveness. Other operatingconditions where the downstream compressor 12B is running can generallyyield a higher pressure differential.

In some embodiments, a diameter of the lubricant transfer conduit 36 canbe relatively smaller in diameter as compared to other lubricanttransfer conduits depending on the application intended. In someembodiments, the relatively smaller diameter can be selected to restricta flow of heat transfer fluid from the lubricant sump 13A to thelubricant sump 13B. In some embodiments, a relatively smaller diameterlubricant transfer conduit 36 can, for example, prevent a pressure inthe lubricant sump 13A and a pressure in the lubricant sump 13B fromequalizing. In some embodiments, this can, for example, maintain apressure differential between the lubricant sumps 13A, 13B to maintain aflow of lubricant between the lubricant sumps 13A, 13B. In someembodiments, the compressors 12A, 12B may be designed to include anoutlet having a diameter designed to fit the relatively larger diameterlubricant transfer conduit. In such embodiments, an adapter (e.g.,adapters 100, 200 shown and described with reference to FIGS. 3A-3Ebelow) can be used to enable the relatively smaller diameter lubricanttransfer conduit 36 to be connected to the compressors 12A, 12B.

FIG. 2 is a sectional view of the flow control device 20, according tosome embodiments. In operation, heat transfer fluid in suction conduit22 (FIG. 1) is provided to the fluid inlet 24 of the flow control device20. In some embodiments, the fluid inlet 24 can be part of the suctionconduit 22.

In general, lubricant in the heat transfer fluid-lubricant mixture ismore concentrated on the perimeter of the fluid inlet 24, and lessconcentrated toward the center of the fluid inlet 24. Lubricant in theheat transfer fluid-lubricant mixture collides with walls 50, 52, andflows toward the fluid outlet 26 which is fluidly connected to theupstream compressor 12A. The lubricant free heat transfer fluid that isdisposed toward a center of the fluid inlet 24 (e.g., along alongitudinal axis of the fluid inlet 24) flows into a nozzle 40 and outfluid outlet 28 to the downstream compressor 12B.

The nozzle 40 extends from the fluid outlet 28 toward the fluid inlet24. In some embodiments, the nozzle 40 has at least a portion with asmaller diameter than the fluid inlet 24. In some embodiments, thenozzle 40 includes at least a portion with a smaller diameter than thefluid inlet 24 such that an inlet to the nozzle 40 is disposed at orabout a central region of fluid flow from the fluid inlet 24. In someembodiments, the nozzle 40 can be sized such that a space is maintainedbetween an inner wall of the fluid inlet 24 and an outer wall of thenozzle 40. In some embodiments, the nozzle 40 extends beyond alongitudinal line extending along a longitudinal axis of the fluidoutlet 26. In some embodiments, the nozzle 40 can be integrally formedwith a suction conduit of the downstream compressor 12B. The nozzle 40is sized to maintain a positive pressure in the lubricant sump 13A ofthe upstream compressor 12A as compared to the lubricant sump 13B of thedownstream compressor 12B. The sizing includes a diameter R2 of thenozzle 40, a length L1 of extension 40A of the nozzle 40, and a lengthL2 of a transition 40B of the nozzle 40. In some embodiments, a value ofthe diameter R2 contributes to the pressure differential. Asillustrated, the radius R1 of the fluid inlet 24 can be larger than aradius R3 of the fluid outlet 28. The fluid outlet 26 has a radius R4that can also be selected to control a flow of heat transfer fluid thatis lubricant rich toward the upstream compressor 12A. Controlling thelocation and cross-sectional area of the nozzle 40, the distributed flowfrom the fluid inlet 24 to the fluid outlets 26, 28 can be controlledfor various compressor conditions (e.g., compressor speeds, etc.). Forexample, controlling an extent to which the nozzle 40 extends toward thefluid inlet 24 as compared to the fluid outlet 26. In the illustratedembodiment, the nozzle 40 and the fluid outlet 26 overlap. In someembodiments, an angle θ of expansion of the nozzle 40 can be selected tocontrol a rate of fluid expansion of the heat transfer fluid flowingthrough the nozzle 40 toward the fluid outlet 28. In general, pressuredrop increases as the angle θ increases.

FIGS. 3A-3E illustrate various views of an adapter 100 and an adapter200 for a lubricant transfer conduit (e.g., the lubricant transferconduit 36 of FIG. 1), according to some embodiments. FIG. 3Aillustrates an isometric view of the adapter 100, according to someembodiments. FIG. 3B illustrates a side view of the adapter 100,according to some embodiments. FIG. 3C illustrates a top view of theadapter 100, according to some embodiments. FIG. 3D illustrates a sidesectional view along the line A-A of the adapter 100, according to someembodiments. FIG. 3E illustrates a side sectional view along the lineA-A of the adapter 100, labeled as 200 because of a modification to alocation of a portion of the adapter. For simplicity of thisspecification, reference will be made generally to the features of FIGS.3A-3E without specific reference to a particular figure unlessspecifically stated otherwise.

The adapter 100 includes a compressor-mating portion 110 and a lubricanttransfer conduit-mating portion 105. The lubricant transferconduit-mating portion 105 has a diameter D2 which is selected based ona diameter of the lubricant transfer conduit 36. The compressor-matingportion 110 has a diameter D1 which is selected based on a diameter ofan outlet from a compressor (e.g., the compressors 12A, 12B). In someembodiments, the diameter D2 can be selected such that the lubricanttransfer conduit 36 can be disposed around the lubricant transferconduit-mating portion 105. The lubricant transfer conduit-matingportion 105 can have an inner diameter D3 which is smaller than thediameter D2.

In the illustrated embodiment, the lubricant transfer conduit-matingportion 105 is disposed relatively lower (e.g., as viewed in FIG. 3B)than a centerline C of the adapter 100. In some embodiments (e.g., FIG.3E), the lubricant transfer conduit-mating portion 105 can be disposedat or about the centerline C. In some embodiments, the lubricanttransfer conduit-mating portion 105 can be disposed relatively higherthan the centerline C of the adapter 100. In some embodiments, thevertical location of the lubricant transfer conduit-mating portion 105can be designed based on maintaining a lubricant level of the lubricantsumps 13A, 13B (FIG. 1) which permits lubricant to flow between thecompressors 12A, 12B (FIG. 1). It will be appreciated that in someembodiments, the compressors 12A, 12B may have a relatively smallerdiameter outlet and in such embodiments, the adapter 100 may not beused.

Aspects

It is noted that any one of aspects 1-7 below can be combined with anyone of aspects 8-9 and/or 10-14. Any one of aspects 8-9 can be combinedwith any one of aspects 10-14.

Aspect 1. A system, comprising:

-   -   first and second compressors, a condenser, an expansion device,        an evaporator, and a flow control device fluidly connected;    -   the first and second compressors being arranged in parallel;    -   the first compressor including a first lubricant sump and the        second compressor including a second lubricant sump;    -   a lubricant transfer conduit fluidly connected to the first        lubricant sump and the second lubricant sump; and    -   the flow control device being disposed between the evaporator        and the first and second compressors, the flow control device        including a fluid inlet and two fluid outlets, a first of the        two fluid outlets being fluidly connected to the first        compressor and a second of the two fluid outlets being fluidly        connected to the second compressor, the second fluid outlet        having a nozzle disposed within a flow passage of the flow        control device such that a space is maintained between an outer        surface of the nozzle and an inner surface of the flow passage.

Aspect 2. The system according to aspect 1, wherein the first compressoris a variable speed compressor and the second compressor is a fixedspeed compressor.

Aspect 3. The system according to any one of aspects 1-2, wherein thefirst and second compressors are scroll compressors.

Aspect 4. The system according to any one of aspects 1-3, wherein thenozzle extends from the second of the two fluid outlets toward the fluidinlet.

Aspect 5. The system according to aspect 4, wherein a longitudinal axisof the second of the two fluid outlets is co-linear with a longitudinalaxis of the fluid inlet.

Aspect 6. The system according to any one of aspects 4-5, wherein alongitudinal axis of the first of the two fluid outlets is perpendicularto the fluid inlet.

Aspect 7. The system according to any one of aspects 4-6, wherein aradius of the fluid inlet is greater than a radius of the second of thetwo fluid outlets.

Aspect 8. A method, comprising:

-   -   separating a flow of a heat transfer fluid and lubricant mixture        into a lubricant rich portion and a lubricant free portion;    -   directing the lubricant rich portion to a suction inlet of a        first compressor, the first compressor being a variable speed        compressor; and    -   directing the lubricant free portion to a suction inlet of a        second compressor, the second compressor being a fixed speed        compressor,    -   wherein the first and second compressors are arranged in        parallel in a heat transfer circuit and the first compressor is        upstream of the second compressor.

Aspect 9. The method according to aspect 9, wherein the separating theflow is completed using a flow control device including a fluid inletand two fluid outlets, a first of the two fluid outlets being fluidlyconnected to the first compressor and a second of the two fluid outletsbeing fluidly connected to the second compressor, the second fluidoutlet having a nozzle disposed within a flow passage of the flowcontrol device such that a space is maintained between an outer surfaceof the nozzle and an inner surface of the flow passage.

Aspect 10. A flow control device for a heating, ventilation, airconditioning, and refrigeration system, comprising:

-   -   a fluid inlet and two fluid outlets, a first of the two fluid        outlets being fluidly connectable to a first compressor and a        second of the two fluid outlets being fluidly connectable to a        second compressor, the second fluid outlet having a nozzle        disposed within a flow passage of the flow control device such        that a space is maintained between an outer surface of the        nozzle and an inner surface of the flow passage.

Aspect 11. The flow control device according to aspect 10, wherein thenozzle extends from the second of the two fluid outlets toward the fluidinlet.

Aspect 12. The flow control device according to aspect 11, wherein alongitudinal axis of the second of the two fluid outlets is co-linearwith a longitudinal axis of the fluid inlet.

Aspect 13. The flow control device according to aspect 11, wherein alongitudinal axis of the first of the two fluid outlets is perpendicularto the fluid inlet.

Aspect 14. The flow control device according to aspect 11, wherein aradius of the fluid inlet is greater than a radius of the second of thetwo fluid outlets.

The terminology used in this specification is intended to describeparticular embodiments and is not intended to be limiting. The terms“a,” “an,” and “the” include the plural forms as well, unless clearlyindicated otherwise. The terms “comprises” and/or “comprising,” whenused in this specification, specify the presence of the stated features,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,integers, steps, operations, elements, and/or components.

With regard to the preceding description, it is to be understood thatchanges may be made in detail, especially in matters of the constructionmaterials employed and the shape, size, and arrangement of parts withoutdeparting from the scope of the present disclosure. This specificationand the embodiments described are exemplary only, with the true scopeand spirit of the disclosure being indicated by the claims that follow.

What is claimed is:
 1. A system, comprising: first and secondcompressors, a condenser, an expansion device, an evaporator, and a flowcontrol device fluidly connected; the first and second compressors beingarranged in parallel; the first compressor including a first lubricantsump and the second compressor including a second lubricant sump; alubricant transfer conduit fluidly connected to the first lubricant sumpand the second lubricant sump; and the flow control device beingdisposed between the evaporator and the first and second compressors,the flow control device including a fluid inlet and two fluid outlets, afirst of the two fluid outlets being fluidly connected to the firstcompressor and a second of the two fluid outlets being fluidly connectedto the second compressor, the second fluid outlet having a nozzledisposed within a flow passage of the flow control device such that aspace is maintained between an outer surface of the nozzle and an innersurface of the flow passage.
 2. The system according to claim 1, whereinthe first compressor is a variable speed compressor and the secondcompressor is a fixed speed compressor.
 3. The system according to claim1, wherein the first and second compressors are scroll compressors. 4.The system according to claim 1, wherein the nozzle extends from thesecond of the two fluid outlets toward the fluid inlet.
 5. The systemaccording to claim 4, wherein a longitudinal axis of the second of thetwo fluid outlets is co-linear with a longitudinal axis of the fluidinlet.
 6. The system according to claim 4, wherein a longitudinal axisof the first of the two fluid outlets is perpendicular to the fluidinlet.
 7. The system according to claim 4, wherein a radius of the fluidinlet is greater than a radius of the second of the two fluid outlets.8. A method, comprising: separating a flow of a heat transfer fluid andlubricant mixture into a lubricant rich portion and a lubricant freeportion; directing the lubricant rich portion to a suction inlet of afirst compressor, the first compressor being a variable speedcompressor; and directing the lubricant free portion to a suction inletof a second compressor, the second compressor being a fixed speedcompressor, wherein the first and second compressors are arranged inparallel in a heat transfer circuit and the first compressor is upstreamof the second compressor.
 9. The method according to claim 9, whereinthe separating the flow is completed using a flow control deviceincluding a fluid inlet and two fluid outlets, a first of the two fluidoutlets being fluidly connected to the first compressor and a second ofthe two fluid outlets being fluidly connected to the second compressor,the second fluid outlet having a nozzle disposed within a flow passageof the flow control device such that a space is maintained between anouter surface of the nozzle and an inner surface of the flow passage.10. A flow control device for a heating, ventilation, air conditioning,and refrigeration system, comprising: a fluid inlet and two fluidoutlets, a first of the two fluid outlets being fluidly connectable to afirst compressor and a second of the two fluid outlets being fluidlyconnectable to a second compressor, the second fluid outlet having anozzle disposed within a flow passage of the flow control device suchthat a space is maintained between an outer surface of the nozzle and aninner surface of the flow passage.
 11. The flow control device accordingto claim 10, wherein the nozzle extends from the second of the two fluidoutlets toward the fluid inlet.
 12. The flow control device according toclaim 11, wherein a longitudinal axis of the second of the two fluidoutlets is co-linear with a longitudinal axis of the fluid inlet. 13.The flow control device according to claim 11, wherein a longitudinalaxis of the first of the two fluid outlets is perpendicular to the fluidinlet.
 14. The flow control device according to claim 11, wherein aradius of the fluid inlet is greater than a radius of the second of thetwo fluid outlets.